Due to recent advances in the technology of laser generation and detection, laser systems for use in battlefield conditions have become more and more prevalent. These laser systems are employed for target illumination and tracking or for ranging. Such laser systems may also be employed for intentional blinding of personnel or sensors. In a particular battlefield setting, there may be numerous laser illuminators operating simultaneously. These laser illuminators may be both from friendly forces and from enemy forces. In particular, combat troops operating in this environment will be subject to uncontrolled illumination by laser radiation. Because of the great radiated power from these laser radiation sources, these personnel require some eye protection from this laser illumination.
There have heretofore been suggested numerous solutions for this problem of laser protection. In particular, in recent years there has been a rise in the use of holographic optical elements as laser protection devices. Holographic optical elements include 3-dimensional interference fringe patterns which diffract light at specified wavelengths. Holographic optical elements are ordinarily constructed employing laser illumination to form the interference fringes within the volume of a photosensitive medium. Upon development of the photosensitive medium, the pattern of the interference fringes is formed within this medium in the form of varying indices of refraction. When light of certain wavelengths enters such a holographic optical element, it is diffracted by the interference pattern therein.
In the case of laser protection eye wear, it is common to form a reflection holographic optical element which reflects incoming radiation at the particular wavelength in a manner making it appear to be a mirror. Because the known laser generators employed in the combat environment includes a relatively limited number of wavelengths, it is possible to form a holographic optical element for each of these wavelengths. The laser protective eye wear becomes, in effect, a reflection filter having a relatively narrow filter band about the expected wavelength of the laser source. It is possible to construct a compound structure including holographic optical elements constructed to reflect differing wavelengths, in order to provide protection for a number of differing laser sources. Because the width of the notch in such holographic optical element reflectors is relatively narrow, normal visibility through such laser protective eye wear, even such eye wear having multiple holographic optical elements for protection against a number of wavelengths, it is relatively unimpaired.
Structures heretofore employed in such laser eye protection devices as goggles or visors do not solve the problems related to the provision of laser protection in a window. In particular, it is known in the art that such holographic optical elements do not provide protection for all angles of incident radiation. These reflection holographic optical elements provide a protection over only a cone of incident angles. Thus, the eye is not protected from laser illumination received at angles of incidence outside this cone. In the case of goggles or visors, it is possible to provide laser protection for greater angles of incidence employing construction geometries related to the expected position of the eye.
In accordance with U.S. Pat. No. 4,637,678, issued to Moss et al. on Jan. 20, 1987 entitled "Holographic Laser Protection Device," a compound holographic optical element structure is taught. A first holographic optical element covers angles of incidence about the normal to the surface of the visor, and a second holographic optical element covers angles of incidence oblique to the surface of the visor. In a second embodiment taught in that patent, the elements in a compound holographic optical element structure offer complementary coverage for angles of incidence from the right and from the left.
In accordance with U.S. Pat. No. 4,830,441 entitled "Holographic Filter Construction for Protective Eyewear," issued May 16, 1989, having the same assignee as the present invention, the geometry of the laser protection eye wear relative to the eye is exploited to provide greater angular coverage. This patent application teaches the use of geometries which are spherically symmetrical about the center of the eye rather than spherically symmetrical about the center of curvature of the protective element as previously taught.
The above teachings relative to goggles and visors cannot be applied to the case of a laser protection window. This is because the eye will not be at a fixed position relative to the laser protection window, contrary to the case of laser eye protection goggles or visors. Therefore, there is a need in the art to provide some manner of laser eye protection through a window such as a vehicle window.